In order to have an antenna radiate RF signals it is necessary to drive an antenna with some sort of power amplifier. Traditional power amplifiers generally have a large power source available in order to drive the antenna. In this case, power consumption by the amplifier is of little concern. Additionally, the space available for the amplifier, such as on a printed circuit board, is ample for nearly any kind of design including those that consist of large inductors or transformers. Also, since many large power amplifiers are not mass manufactured, the ease of manufacture is of little concern.
Traditionally, the output stage of a power amplifier that drives an antenna consists of bipolar transistors because their output impedance is low and they are capable of sourcing or sinking large currents. The large current necessitates transistors that tend to be large in size, typically requiring heat sinks to dissipate heat in order to maintain proper operation. Additionally, the other components of this circuitry, including large transformers and inductors, consume a great deal of space or circuit area. In order to maximize the power output to the antenna, the bipolar transistors are often driven in a push-pull fashion such that the bipolar transistor driving one side of the antenna is close to being 180 degrees out of phase with the other bipolar transistor driving the opposite side of the antenna. And, the impedance of the bipolar output stage is matched with the impedance of the antenna to maximize the power output. Because of the nature of bipolar transistors, circuit trimming devices such as variable resistors and capacitors are often used within the power amplifier circuit in order to properly tune the output stage to drive at the proper frequency and power output levels. Also, in operation, the bipolar transistors provide a substantially fixed, rather than adjustable power output.
Diode Metal Oxide Semiconductor (DMOS) devices are alternatively used in the place of bipolar transistors in the output stage of power amplifiers. These devices have limitations that are similar to bipolar devices, including requiring a heat sink or other heat-dissipating device when at high output power requirements. These heat-dissipating devices tend to be large and expensive, taking up valuable space on a printed circuit board and increasing manufacturing cost of the power amplifier.
Today there are radio frequency (RF) systems that require high volume manufacture at low costs using a minimal amount of circuit components and printed circuit board (PCB) area. One such RF system is a radio frequency identification (RFID) system. In order to make the RFID systems in high volume at low cost, it is desirable to improve the ease of manufacture of a power amplifier. Additionally, it is desirable to eliminate the circuit trimming devices, the large circuit components and the heat sinks ordinarily used in power amplifier design. Furthermore, it is desirable to lower manufacturing costs by developing a power amplifier that requires minimal PCB area by using smaller off the shelf components. And, because of the prevalence of portable systems, it is desirable to reduce the power consumption so that battery-operated systems have a longer battery replacement cycle. Traditional bipolar and DMOS power amplifiers have obvious shortcomings in providing the desired level of manufacturablity, portability, power efficiency and flexibility.
Therefore a need exists for an adjustable radio frequency power amplifier that is readily mass manufactured, power efficient, flexible and compact in size.